Exam Questions (Mostly motors)

[michael071284]

Hi just doing a mock exam at the minute and have a few questions I cant answer. Can anyone telll us the answers

1 - All cable connections should be accessible for inspection exept cables

a- swimming pools
b- overhead lines
c- enclosed in non flammable material
d- sheathed in asbestos

2 - In a star connected motor the supply voltage is connected

a- to all 6 connections
b- across 2 windings in series
c- across 2 winding in paralell
d- across 3 windings in series

3 - In a delta connected motor

a- the supply is connected across each winding
b- the supply is connected to one end of each winding
c- the neutral point of the windings is at zero voltage
d- the neutral point of the windings are connected to the neutral of the supply

4 - Which of the following statments is correct

a- When a motor is single phasing the current and torque is low
b- In a star connected motor 1 winding will burn out if the motor is left single phasing
c- In a delta connected motor on winding will burn out and the other 2 windings over heat if the motor is left single phasing
d- Once a motor attains its rated speed single phasing will not cause any damage to the motor windings

5 - When a motor is operated through a star/delta starter

a- The starting current and torque is reduced
b- The starting torque and current is increased
c- The starting current is reduced and the tourque is incresed
d- The starting current is doubled and the starting torque is halved

6 - A single phase motor hums but the shaft does not rotate. This could be due to

a- One of the windings open circuit
b- The switched siezed in the on position
c- Both of the windings open circuit
d- No load being present on the motor

7 - When a motor is connected in delta and the resistance of each winding is 6R what reading will you get if you measure accross U1 and W1

a- 6R
b- 12R
c- 4R
d- 3R

 

[ericmark]

Hi just doing a mock exam at the minute and have a few questions I cant answer. Can anyone tell us the answers

1 - All cable connections should be accessible for inspection except cables

a- swimming pools
b- overhead lines
c- enclosed in non flammable material
d- sheathed in asbestos

Non of the above 526.3 Every connection shall be accessible for inspection, testing and maintenance, except for the following:
(i) A joint designed to be buried in the ground
(ii) A compound-filled or encapsulated joint
(iii) A connection between a cold tail and the heating element as in ceiling heating, floor heating or a trace heating system
(iv) A joint made by welding, soldering, brazing or appropriate compression tool
(v) A joint forming part of the equipment complying with the appropriate product standard.

2 - In a star connected motor the supply voltage is connected

a- to all 6 connections
b- across 2 windings in series
c- across 2 winding in parallel
d- across 3 windings in series

Since it needs 3 connections the answer must be (d) although not really is series very bad wording.

3 - In a delta connected motor

a- the supply is connected across each winding
b- the supply is connected to one end of each winding
c- the neutral point of the windings is at zero voltage
d- the neutral point of the windings are connected to the neutral of the supply

No neutral point so that’s c and d out and so (a) has to be answer.

4 - Which of the following statements is correct

a- When a motor is single phasing the current and torque is low
b- In a star connected motor 1 winding will burn out if the motor is left single phasing
c- In a delta connected motor on winding will burn out and the other 2 windings over heat if the motor is left single phasing
d- Once a motor attains its rated speed single phasing will not cause any damage to the motor windings


One wonders if the question should have been “Which of the following statements is NOT correct”? If so I would select (d) as NOT being correct!

5 - When a motor is operated through a star/delta starter

a- The starting current and torque is reduced
b- The starting torque and current is increased
c- The starting current is reduced and the torque is increased
d- The starting current is doubled and the starting torque is halved

It should say compared with what other method. However compared with direct on line then (a) would be correct.

6 - A single phase motor hums but the shaft does not rotate. This could be due to

a- One of the windings open circuit
b- The switched seized in the on position
c- Both of the windings open circuit
d- No load being present on the motor

I would select (a)

7 - When a motor is connected in delta and the resistance of each winding is 6R what reading will you get if you measure across U1 and W1

a- 6R
b- 12R
c- 4R
d- 3R

A 12 ohm and a 6 ohm resistance in parallel will 4 ohms so answer is (c).

I would agree some of the questions do leave a lot to be desired and I can understand problems with 4 and 1. And with 5 there should have really been more information although I would think most people would assume it was compared with direct on line and not with auto transformer, resistor, or inverter/soft-start controls.
6 does take a little thought but should not really present a problem and 2, 3, and 7 are relatively straight forward. With 7 one does have to think what U, V, W and X, Y, Z, mean as to how connected to a motors winding and maybe it should have had a diagram with it.

 

[electronicsuk]

Who wrote those questions? If it was your college tutor, he needs sacking.

 

[ericmark]

I have been reconsidering 4.

a- When a motor is single phasing the current and torque is low
Can't be right as current will increase in the two supplied windings.

b- In a star connected motor 1 winding will burn out if the motor is left single phasing
Well correct as once one winding burns out the other one will lose supply.

c- In a delta connected motor one winding will burn out and the other 2 windings over heat if the motor is left single phasing

This could also be correct as one winding will have full 400 volt and other two just 200 volt but since motor will not be turning they will not be cooled and will over heat.

d- Once a motor attains its rated speed single phasing will not cause any damage to the motor windings

This is not true although where multi-motors are running they can produce a third phase so under light load it may appear to be correct.

So even if a "NOT" has been missed out of question there is still not a 100% correct answer.

 

[ericmark]

Who wrote those questions? If it was your college tutor, he needs sacking.

I would agree. Very poor

 

[Spark123]

I'd have said the swimming pool one for above, as they need to be submersible so resin filled?

 

[ericmark]

I'd have said the swimming pool one for above, as they need to be submersible so resin filled?

Sure not (b) all needed is good pair of binoculars and a helicopter! Access without Faraday cage suit and equalising rod may produce sparks!

 

[michael071284]

thanks for the help lads, im in ireland so maybe the answer to question one is different here. yea its questions from past exams

 

[Spark123]

Bog standard overhead lines are covered by BS7671, overhead transmission lines are not.

 

[matt1e]

regarding the motor questions

2 - In a star connected motor the supply voltage is connected

a- to all 6 connections
b- across 2 windings in series
c- across 2 winding in parallel
d- across 3 windings in series

I would say B as there are 2 windings in series between any 2 phases but agree with others, poorly worded

3 - In a delta connected motor

a- the supply is connected across each winding
b- the supply is connected to one end of each winding
c- the neutral point of the windings is at zero voltage
d- the neutral point of the windings are connected to the neutral of the supply

answer a

4 - Which of the following statements is correct

a- When a motor is single phasing the current and torque is low
b- In a star connected motor 1 winding will burn out if the motor is left single phasing
c- In a delta connected motor on winding will burn out and the other 2 windings over heat if the motor is left single phasing
d- Once a motor attains its rated speed single phasing will not cause any damage to the motor windings

the answer that comes closest to being true is c) as one winding will be carrying over 2 and a half times its rated current whilst single phasing but the question should read
In a delta connected motor one winding may burn out and the other 2 windings over heat if the motor is left single phasing

b could also be true depending on how much the motor is loaded and whether it has stalled,in which case a winding could burn out due to overheating,
its a P'ss poorly worded question
I'd prob still pick c) though

5 - When a motor is operated through a star/delta starter

a- The starting current and torque is reduced
b- The starting torque and current is increased
c- The starting current is reduced and the torque is increased
d- The starting current is doubled and the starting torque is halved

answer a)

6 - A single phase motor hums but the shaft does not rotate. This could be due to

a- One of the windings open circuit
b- The switched seized in the on position
c- Both of the windings open circuit
d- No load being present on the motor

answer (a)

7 - When a motor is connected in delta and the resistance of each winding is 6R what reading will you get if you measure across U1 and W1

a- 6R
b- 12R
c- 4R
d- 3R

answer (c).

 

[Stoday]

I would put it down to a bad translation from the Irish.

As for Q4, the answer's none of them because the single phasing protection operates.

 

[holmslaw]

..

 

[ericmark]

I would not think it's down to being Irish but more like some one has miss copied the questions and missed out bits.

For example with

5 - When a motor is operated through a star/delta starter

a- The starting current and torque is reduced
b- The starting torque and current is increased
c- The starting current is reduced and the tourque is incresed
d- The starting current is doubled and the starting torque is halved

Both torque and increased are misspelt which points to poor copying somewhere and star/delta starters draw more current than auto transformer, Resistor, Inverter, and soft-start methods only with comparison with direct on line would the current be reduced.

So maybe it is referring to the change over between star and delta and as it changes over then b) would be correct. Most sample papers do also give the answers expected and it would be interesting to see the answer given a) or b)!

matt1e has pointed out that with

2 - In a star connected motor the supply voltage is connected

a- to all 6 connections
b- across 2 windings in series
c- across 2 winding in paralell
d- across 3 windings in series

b) could also be consider correct as each pair of supply windings will connect across two windings in series not three in zig zag formation.
However I would consider d) was more likely the answer required as all three windings will be connected to all three phases.

However when referring to series and parallel connections with motors we are normally referring to multi-speed motors with possible 12 windings not just 3 and the question really does not make sense.

I think the idea of an exam on Electrics is to test ones knowledge of Electrics and not ones powers of comprehension. And if a question has to be re-read to understand what it is asking then it is a poor question. There is of course the pedantic answers and when asked if E = I x R was Ohms law a student answer was "no". As that was the formula for Ohm's law, not the law itself, which states that the current through a conductor between two points is directly proportional to the potential difference or voltage across the two points, and inversely proportional to the resistance between them. Technical he was correct and he answered that way because the lecturer was taking great pains to point out how one should read the question carefully to see exactly was it was asking first before answering.

Even with the C&G2382 exam there are some questions which one at first glance seem to have two answers. For example:- "A person with technical knowledge or sufficient experience to enable him/her to avoid dangers which electricity may create." does this refer to Skilled or Competent person and I would say it covers both but a Competent person also is able to protect others and at first glance I would have said it is a poor question. However the Exam is testing your ability to read and comprehend a specific book not ones' electrical knowledge so it is a fair question.

With this in consideration if the questions were extracted from a set book rather than general electrical questions maybe one would better understand and answer questions?

 

[michael071284]

these are questions taken directly from a mock exam and i have not missed anything out, have the paper here in front of me and they are the same